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Selecting a Silicon Creations PLL For Your System

Silicon Creations offers a large selection of PLL architectures, functions, and performance trade-offs which can make the task of choosing the correct PLL for a given system seem daunting. Fortunately, this task can be greatly simplified when a few basic guidelines are followed. If after following these guidelines the correct choice is still not clear, a Silicon Creations expert PLL designer will be happy to help.

1. Determine the right VCO architecture:
   Silicon Creations currently offers two VCO architectures:
   • Ring Oscillator
   • Single LC Tank Oscillator
   • Multi-Core LC Tank Oscillator

   If a few basic system specifications are known, the best VCO architecture can usually be chosen. The following table gives some general guidelines that apply in most modern processes. Of course, there are always exceptions to these guidelines so if your system requires a special case, Silicon Creations will be happy to help develop a custom solution.

   In the table below, each column compares the three VCO architectures for a given specification. Start with the specifications that are most important in your system and then move to less important specifications. As you move through, some VCO architectures will clearly appear more optimal while others may get eliminated. If none of the VCO architectures appear optimal for your system, please contact Silicon Creations and an expert PLL designer will help find a solution.

   VCO Architecture	Minimum PLL Current Consumption	Minimum PLL Area	RMS Long Term Jitter	Frequency Range
   Ring	<1mA	150u x 300u	< 5ps with 1MHz BW	2:1
   LC Tank	20mA	700u x 1000u	< 1ps with 50kHz BW	+/-10%
   Multi-Core LC Tank	25mA	1mm x 1mm	< 1ps with 50kHz BW	2:1

   Note that the numbers given in the previous table are average values for most modern day (130nm-45nm) processes. The intent is to show the relationship between the various VCO architectures and give rough estimates of what can be expected for each design.

2. Choose integer or fractional
   In an integer based PLL, the minimum frequency step size is equivalent to the reference clock frequency. Therefore, if a fine frequency step is needed a fractional PLL may be the best solution. Also, if only a single crystal frequency is available and multiple unrelated output frequencies are needed, then a fractional PLL provides a nice solution since a single PLL instance can be used and controlled digitally to generate the desired outputs.

   If cost or area are the major concerns, an integer PLL can also be used for fine frequency resolution by dividing down the reference clock frequency. The problem with this is that to maintain stability the loop bandwidth must also be lowered resulting in significantly increased phase noise and long term jitter. However, if short term jitter (period or cycle-to-cycle) is the only concern this may be acceptable.

   Silicon Creations uses an advanced delta sigma modulator architecture to generate fractional clocks so quantization noise is highly attenuated. The only real trade-off of going to fractional mode is slightly increased area and complexity.
    

3. Choose accessories:
   Additional options include:
   • Spread Spectrum
   • Input/Output Frequency Dividers
   • De-skew Logic
   • Lock Detection
   • Differential Output Clocks
   • Voltage Regulator
   • ESD

   The accessories needed for a specific PLL are usually obvious from the system requirements and frequency plan. For example, if the function of the PLL is to de-skew two clock domains then de-skew logic will likely be needed but spread spectrum clocking will not. If only a 3.3V supply is available in the system or if the core supply is shared with a large digital block then a voltage regulator may be needed. If the PLL is located in the IO ring, ESD protection may be needed.

   If it is not clear from the system requirements which accessories are necessary, please contact Silicon Creations and an expert PLL designer will help find a solution.

   Still not sure what PLL you need, please fill out as much as possible of the Custom PLL Request Form and Silicon Creations will propose a solution.